This invention relates to a method and apparatus for forming a flat film made of synthetic resin into a tube, filling the tube with a liquid or highly viscous material continuously while the tube is being formed, and producing a projectile-shaped package constituted by the filled film tube.
Projectile-shaped packages such as sausages or cheese sticks in which a tubular film is filled with the particular foodstuff are well-known in the art. A known method of manufacturing a package of this kind includes the steps of forming a flat film of synthetic resin into a tubular shape, forming a tube by fusing overlapped longitudinal side edges of the film while the film is being formed into the tubular shape, filling the interior of the tube continuously with a material to be packaged to produce a tubular body formed continuously by being stuffed with the material, squeezing the traveling tubular body intermittently at a predetermined time interval by a pair of rotating cylindrical rollers to form flattened portions by partially pushing aside the material where the tubular body is squeezed, constricting each flattened portion transversely of the tubular body, forcibly deforming an aluminum U-shaped wire clip at two points on the constricted portion to seal the same, and severing the constricted portion between the two wire clips.
The aforementioned sealing method which relies upon the wire clips does not provide a perfect seal and is a source of various problems. An expedient for providing a perfect seal is therefore desired. Methods of achieving this include effecting an ultrasonic seal between the two wire clips, as disclosed in the specifications of Japanese patent application Laid-Open (KOKAI) Nos. 47-46289, 46-46282, and providing an ultrasonic seal directly without using wire clips at all, as taught in the specification of Japanese patent application Laid-Open (KOKAI) No. 59-26424. Still, problems remain, as will now be set forth.
(1) In an arrangement where the film is clamped between an anvil and ultrasonic horn to effect a seal by the application of ultrasonic waves, a large clamping force is required. However, if a large clamping force is applied with an impact, the anvil and horn cause pinholes to form in the film, and in many cases the pinholes remain even after the sealing operation. This problem becomes pronounced at higher operating speeds.
(2) Despite the clamping of the film by the anvil and horn and the application of the ultrasonic waves, the film is clamped for only a short period of time. Consequently, when the electrode is simply detached after the sealing operation, the terminus of the seal easily develops a pinhole caused by the internal pressure of the filled material or when a force is mechanically applied to the film.
(3) When the film is constricted solely by upper and lower constricting devices, the constricted film between the devices spreads and unfolds. Then, when the film is clamped by the anvil and ultrasonic horn between the two constricting devices, there is a large disparity in the number of film overlaps from one location to another. As a result, pinholes readily form when ultrasonic sealing is applied. When it is attempted to achieve a perfect seal up to where there is the minimum number of overlaps, namely where only a two-ply overlap, the overall result is over-sealing. This results in tearing at the seal ends and the formation of pinholes. If the application of the ultrasonic waves is weakened, on the other hand, then the two-ply overlap will not be fused completely and the package will leak from this region.
(4) When a cutter is introduced to sever the film during sealing, the cutting force stretches the end of the seal and causes a pinhole to form. In addition, the cutting operation is difficult to perform. If the cut is made before sealing, the folds in the film will become undone and pinholes will form for the reasons set forth in (3) above.